The present invention relates to a mechanism for detecting regions of a specified color in a color television signal and specifically to systems for detecting a specified color for warning operators of equipment to the presence, in the field of view of a television camera, of objects or personnel exhibiting a specified color.
An essential part of the operation of industrial equipment, such as construction machinery is the implementation of safety procedures and mechanisms to prevent accidents. Safety precautions are particularly important when operating large machines with massive or rapidly moving parts. This is especially true when such machines are used on busy streets in the presence of workers and passers-by.
A typical safety procedure followed by operators of mobile construction machinery, such as cranes and trucks, is to rely on spotters and visual checks. The spotters and the operator check a hazard zone, a zone within reach of moving elements of the machinery, to determine if it is clear of unauthorized personnel.
Sole reliance on visual checking entails a risk of overlooking people in the hazard zone. An automated means for detecting and warning of people in the hazard zone can help to prevent the danger posed by such oversight. Such a warning system could be implemented to supplement or replace visual checking.
One known device for providing such automated warning systems is disclosed in laid-open Japanese patent application No. 5-63500. A color television camera, which produces a National Television System Commission (NTSC) signal, is trained on the hazard zone. A circuit monitors the signal from the color television camera to detect the presence of the specified color in the NTSC signal when an object of the specified color enters the hazard zone.
Referring to FIG. 11, in the NTSC signal format, hue is represented by a phase-modulated color signal whose instantaneous value can be transformed, by a known relationship, into a color vector on a Maxwell color chart. The color vector originates at the position 0 and points in a direction which indicates the hue. For example, when the color vector points to 10 o'clock, the color represented is green. When it points to 2 o'clock, the color represented in red. Between these colors are their additive mixtures grading between green and red. For example, an equal additive mixture of green and red produces yellow. The result of additive mixtures can be found by vector addition of the color vectors of the component hues. Thus, a unit vector representing red, vectorially added to a unit vector representing green, results in a unit vector representing yellow.
The magnitude of the color vector indicates the degree of saturation. Thus, a vector of zero magnitude represents zero saturation, or white. The luminance or intensity of the image is carried by a separate signal embedded in the NTSC format known as the luminance signal.
In a color television, the NTSC signal is transformed by circuitry into three primary color signals, one each for the colors red, green and blue. This is done for the purpose of producing an image on a color cathode ray tube. The relative proportion of the signals representing each of the primary colors represents both the magnitude and direction of the color vector. For example, if all three signals are equal in magnitude, the color represented is white, since vectorial addition produces a null vector. If the blue signal is zero and the green and red signals are equal, yellow results. If the blue signal then becomes non-zero, the saturation of the yellow falls until, finally the blue signal reaches the value of the equal red and green signals, whereupon white is represented.
The circuit of the known device first automatically controls the gain of the color signal levels to maintain a constant amplitude of their sum. The signals are analyzed to determine if the color vector falls in the zone indicated by shading in FIG. 11. The shaded zone is defined by the following parameters:
1) the range in which the magnitude of the yellow component of the color vector is at or greater than a specific value, E2. PA1 2) the range in which the magnitude of the component perpendicular to the yellow component is less than another specified value, E3. PA1 3) the luminance signal is at or greater than a specified value. A color signal following all three limits is defined as yellow in the foregoing prior art device.
In addition, the following definitional limit is applied:
The portion of the chrominance element of the NTSC signal that distinguishes yellow is limited to a band of 0.5 MHz. Therefore, the above circuit, which extracts specified color elements cannot detect yellow in the color-difference signal when the desired object area on the screen is too small. This is because the time during which the chrominance signal represents yellow is too short to be represented by frequencies below 0.5 MHz.